1. Field of Invention
This invention relates to a microwave filter and more particularly to a dual mode filter.
2. Description of the Prior Art
A man made satellite in an orbit about the earth usually has a payload that either serves as a communication relay station or provides data related to weather conditions on the earth. Launching the satellite into the orbit may be difficult and expensive when either the size or the weight of the payload becomes excessive. Therefore, it is desirable to make the payload as small and as light as possible.
The payload may include a transponder that transmits a modulated signal at a frequency within one of twelve signal channels in response to a signal received from a ground station. The signal channels are defined by pass bands of twelve dual mode filters that are included in the transponder. The pass bands may be within a broad band that typically extends from 3.7 GHz to 4.2 GHz, each of the pass bands having a 40 MHz bandwidth.
A dual mode filter is usually comprised of a circular waveguide formed of a cylinder having one or more coaxial cylindrical cavities in tandem, each supporting a pair of TE.sub.11 modes of propagation therethrough of electromagnetic energy. Usually, if not always, a metal enclosure, known as a coaxial transition assembly, is connected to the end cavity of the dual mode filter via a coupling obstacle. The output of the dual mode filter is provided via a connector mounted upon the coaxial transition assembly. The transition assembly is undesirable because of its size and weight.
In the transponder referred to above, the twelve filters have inputs connected to a known arrangement of rectangular waveguides, referred to herein as a signal manifold system. Signals associated with all of the signal channels are applied to the filter inputs via the manifold system. Terminal conditions at the filter inputs, caused by the rectangular waveguides of the manifold system, usually result in the twelve filters supporting a mode of propagation within a parasitic band (typically two MHz wide). The parasitic band is separated from the upper frequency of the broad band by a small spectrum of frequencies, determined by the diameters of the cavities of the dual mode filters, such spectral separation typically being on the order of 35 MHz. Therefore, the manifold system may cause undesired signals in the parasitic band to pass to the outputs of the twelve filters.
A low pass filter may be connected to the manifold system to prevent parasitic signals from being applied to the filter inputs. However, because the broad band and the parasitic band have the small spectral separation, the low pass filter degrades signals within pass bands of some of the twelve filters.
There is a need, thus, for a band pass filter that is small, light and does not pass parasitic signals.